The invention is a continuous excavation/demolition system based upon the controlled fracturing of hard competent rock and concrete through the controlled application of a high-pressure foam-based fluid in predrilled holes.
For over a century explosive blasting has been the primary means used for the excavation of hard rock and often the demolition of concrete structures. In recent years several small-scale methods employing small explosive or propellant charges or specialized mechanical and hydraulic loading means have been proposed as alternatives to conventional blasting. Conventional blasting is limited in that it requires special precautions due to the use of explosives and that it can cause excessive damage to the rock or concrete being broken. The smaller scale specialized techniques, while finding many niche applications, have been limited in their ability to break harder rocks or in having undesirable operating characteristics. For example, the small-charge explosive and propellant techniques still generate significant airblast and fly rock.
Efforts to develop alternatives to conventional explosive excavation and demolition have included water jets, firing high velocity slugs of water into predrilled holes, rapidly pressurizing predrilled holes with water or propellant generated gases, mechanically loading predrilled holes with specialized splitters, various mechanical impact devices and a broad range of improvements on mechanical cutters. Each of these methods may be evaluated in terms of specific energy (the energy required to excavate or demolish a unit volume of material), their working environment, their complexity, their compatibility with other excavation operations, and the like.
The excavation of hard rock for both mining and civil construction and the demolition of concrete structures are often accomplished with conventional explosives. Due to the very high pressures associated with explosive detonation these operations are hazardous, environmentally disruptive, require considerable security, protection of nearby personnel and equipment and must often be applied on an inefficient cyclic basis (as in conventional drill-blast-ventilate-muck operations).
Efforts to develop continuous and more benign excavation/demolition methods have been ongoing due to persistent problems in the industry. The PCF (Penetrating Cone Fracture) method using small propellant charges has proven the most promising to date. However, the PCF method is most limited as it still generates considerable airblast and fly rock, and as the propellant reaction gases may be comprised of over 50 percent carbon monoxide, a poisonous gas. The strength of the PCF method as compared to the other small-charge, electrical discharge and water cannon methods lies in that the propellant gases are able to maintain sufficient pressure for fracturing as the fracture system grows and increases in volume. It is the continuous and maintained pressurization of the developing fractures that enable the PCF method to work efficiently.
The present invention uniquely overcomes the limitations of all the above excavation/demolition methods. The present invention shows that the proper pressurization of preferred or controlled fractures is the most efficient way to excavate or demolish rock and concrete.
A preferred excavation/demolition method of the invention has the ability to pressurize a controlled fracture (or system of fractures) in such a manner that pressures to just propagate the fractures (without over pressurizing them) are maintained.
A fluid to achieve such controlled pressurization has a viscosity such that the fracturing process occurs over a longer duration and thus at lower pressures. The fluid is able to store energy that can be used to maintain a desired pressure as the fluid expands into the developing fracture system. The generation, control and application of such a preferred fluid is the subject of the current invention. The current invention or method is based upon using high-pressure foam as the fracturing medium. This method is referred to as Controlled-Foam Injection (CFI) fracturing. The Controlled-Foam Injection method overcomes the limitations of the existing explosive, propellant, water and steam fracture pressurization methods.
In a preferred embodiment, the invention is a continuous excavation/demolition system based upon the controlled fracturing of hard competent rock and concrete through the controlled application of a high-pressure foam-based fluid in predrilled holes.
The present invention provides both method and means for maintaining the fracture pressurization needed for efficient fracturing without the adverse aspects of the explosive and propellant based methods.
A preferred fluid may be generated with commercially available pumps and applied to the controlled pressurization of predrilled holes by simple and straight forward valving means. A preferred foam, herein considered preferably to be a two-phase mixture of a liquid and a gas, may have a viscosity several orders of magnitude higher than a gas. Foam escapes from a developing fracture system much more slowly than a gas. With a much slower escape of the fracture pressurizing media, the pressures required to initiate, extend and develop the desired fractures is much lower than if a gas alone is used.
The use of a high viscosity liquid (e.g. water) alone is not sufficient because the relatively incompressible liquid will rapidly lose pressure as the fracture volume increases with fracture growth. A foam in contrast maintains the pressures for efficient fracturing due to the expansion of the gaseous phase of the fluid. Foam has the ability to provide the pressures for efficient controlled fracturing without requiring the excessively high pressures associated with explosives, propellants, water cannons or electrical discharge.
The successful application of a foam based controlled fracturing system of the invention provides the means for generating a foam of certain desirable physical properties; the means to deliver the foam to the bottom of a predrilled hole on an as needed basis, in terms of pressure, pressure time behavior and volume; and the means to limit or control the escape of foam around the barrel or other device used to deliver the foam to the hole bottom.
These and further and other objects and features of the invention are apparent in the disclosure, which includes the above and ongoing written specification, with the claims and the drawings.